There is difficulty in maintaining performance improvements in devices of deeply submicron generations. One general approach for improving performance is to try to increase carrier (electron and/or hole) mobilities. A promising avenue toward better carrier mobility is to apply tensile or compressive stress in the semiconductor channel regions. Typically, it may be preferable to have the channel of electron conduction type devices, such as NFET, in tensile stress, while to have the channel of hole conduction type devices, such as PFET, in compressive stress. In the case of the more common planar devices a method known in the art for stressing channels is the deposition of so called stress liners. Stress liners are insulating layers covering the devices, which layer are under stress, and then impart the stress onto the device channels. It is known that the stress distribution is such that if a stressing layer which covers a planar FET, is, for instance, in tensile stress, then the channel of the FET will also be under tensile stress, while the source and drain of the same device would be under compressive stress.
Applying stress to non-planar, three dimensional (3D) FETs, such as a FinFET, or Tri-Gate device, may be more difficult. Stress liners commonly used for planar devices are not very efficient, partly because of the 3D nature of the device, and partly because of the shrinking of the device pitch as technology progresses. Smaller device pitch leads to thinner stress liners, and that results in less channel stress. One way to increase the stress coupling to FinFET channel regions is to recess, namely etch down, the source/drain area of the fins or epitaxially grow material above and below the source/drain area of the fins.
In some semiconductor devices, gate pairs may be separated by a narrow pitch and other gate pairs may be separated by a wider pitch. Currently, during epitaxy growth, it is difficult to ensure that epitaxy merges the fins separated by the wider pitch to provide the strain benefit while also limiting epitaxy overgrowth between the fins separated by the narrow pitch.